In recent years, research and development have been carried out actively for electroluminescent light-emitting elements. A basic structure of these light-emitting elements is that a light-emitting substance is sandwiched between a pair of electrodes. The application of voltage to this element enables the light-emitting substance to emit light.
Since such light-emitting elements are of a self-luminous type, they have advantages such as having higher visibility compared with liquid crystal displays and no need of back lights. Therefore, these light-emitting elements are suitable for flat panel display elements. Another significant advantage is that such light-emitting elements can be manufactured to be thin and light-weight. Another feature is that these light-emitting elements have extremely high response speed.
Since these light-emitting elements can be formed in a film form, planar light emission can easily be obtained by forming elements with large areas. Planar light emission is hard to obtain from point-light sources such as incandescent lamps and LEDs, or line sources such as fluorescent lights. Therefore, these light-emitting elements have a high utility value as surface illuminants. The surface illuminants can be applied to lighting and the like.
Electroluminescent light-emitting elements are largely classified into two types according to whether the light-emitting substance is an organic compound or an inorganic compound. Here the light-emitting substance which is the organic compound is described.
When the light-emitting substance is an organic compound, the application of voltage to the light-emitting element makes electrons injected from an electrode into a layer containing the light-emitting organic compound, holes injected from the other electrode into the layer containing the light-emitting organic compound, and then electric current flows. Then, the recombination of these carriers (electrons and holes) excites the light-emitting organic compound. The light-emitting organic compound emits light in returning to a ground state. Because of such mechanism, such a light-emitting element is referred to as a light-emitting element of a current excitation type.
Excitation states of an organic compound can be classified into two types: a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio of the singlet excited state (S*) and the triplet excited state (T*) in a light-emitting element is considered to be S*:T*=1:3.
A ground state of a light-emitting organic compound is usually a singlet state. Therefore, light emission in returning to a singlet ground state from a singlet excited state (S*) is referred to as fluorescence since it is caused by electronic transition between the same multiplets. On the other hand, light emission in returning to a singlet ground state from a triplet excited state (T*) is referred to as phosphorescence since it is caused by electronic transition between different multiplets. In most of the compounds which emit fluorescence (hereinafter referred to as “fluorescent compounds”), only fluorescence is observed and phosphorescence is not at room temperature. Therefore, the maximum value of the internal quantum efficiency (the ratio of photons to be generated with respect to injected carriers) of a light-emitting element containing a fluorescent compound is said to be theoretically 25%, on grounds that S*:T*=1:3.
On the other hand, by using a compound which emits phosphorescence (hereinafter referred to as a “phosphorescent compound”), an internal quantum efficiency of 75 to 100% is possible in theory. In other words, it is possible to achieve light emission efficiency which is three to four times that of a fluorescent compound. For such a reason, a light-emitting element using a phosphorescent compound is proposed in order to achieve a highly efficient light-emitting element (for example, refer to Non-Patent Document 1: Tetsuo TSUTSUI et al., “Japanese Journal of Applied Physics” vol. 38, 1999, pp. L1502-L1504)
As a phosphorescent compound, a complex containing iridium (Ir) as a central metal is used in general as in Non-Patent Document 1. However, iridium is a noble metal and exists in crusts in extremely small amounts. Accordingly, there arises a problem of resource depletion of iridium, as light-emitting devices and electronic appliances using light-emitting elements come into wide use. In addition, in order to reduce adverse impact on the environment, methods for reusing iridium are needed.